THE SECOND EPISTLE GENERAL OF PETER - Chapter 3 - Verse 6

Verse 6. Whereby. di wn. Through which, or by means of which.
The pronoun here is in the plural number, and there has been much
difference of opinion as to what it refers. Some suppose that it refers
to the heavens mentioned in the preceding verse, and to the fact that the
windows of heaven were opened in the deluge, (Doddridge;) others that
the Greek phrase is taken in the sense of (dio) whence.
Wetstein supposes that it refers to the "heavens and the earth." But the
most obvious reference, though the plural number is used, and the word
water in the antecedent is in the singular, is to water. The fact
seems to be that the apostle had the waters mentioned in Genesis
prominently in his eye, and meant to describe the effect produced by
those waters. He has also twice, in the same sentence, referred to
water" out of the water and in the water." It is evidently to
these waters mentioned in Genesis, out of which the world was
originally made, that he refers here. The world was formed from that
fluid mass; by these waters which existed when the earth was made, and
out of which it arose, it was destroyed. The antecedent to the word in
the plural number is rather that which was in the mind of the writer, or
that of which he was thinking, than the word which he had used.

The world that then was, etc. Including all its inhabitants.
Rosenmuller supposes that the reference here is to some universal
catastrophe which occurred before the deluge in the time of Noah, and
indeed before the earth was fitted up in its present form, as described
by Moses in Ge 1. It is rendered more than probable, by the
researches of geologists in modern times, that such changes have
occurred; but there is no evidence that Peter was acquainted with them,
and his purpose did not require that he should refer to them. All that
his argument demanded was the fact that the world had been once
destroyed, and that therefore there was no improbability in believing
that it would be again. They who maintained that the prediction that the
earth would be destroyed was improbable, affirmed that there were no
signs of such an event; that the laws of nature were stable and uniform;
and that as those laws had been so long and so uniformly unbroken, it was
absurd to believe that such an event could occur. To meet this, all that
was necessary was to show that, in a case where the same objections
substantially might be urged, it had actually occurred, that the world
had been destroyed. There was, in itself considered, as much
improbability in believing that the world could be destroyed by water
as that it would be destroyed by fire, and consequently the objection
had no real force. Notwithstanding the apparent stability of the
laws of nature, the world had been once destroyed; and there is,
therefore, no improbability that it may be again. On the objections
which might have been plausibly urged against the flood,
See Barnes "Heb 11:7".